In an optical heterodyne detection communication system, a long distance and high density transmission can be performed because a receiving sensitivity is much increased as compared to a direct detection communication system in which light intensity is modulated, and because a frequency utilizing efficiency is high as described in "Coherent Optical Fiber Transmission Systems" reported in IEEE QE-17, No. 6, June 1981, pp. 919-935 by Y. Yamamoto and T. Kimura.
In the coherent light communication, signal light supplied from an optical transmitter and local oscillation light radiated from a local oscillation light source contained in an optical receiver are combined to produce an intermedeate frequency signal which is a beat signal corresponding to a frequency difference between the signal light and the local oscillation light. In an optical detector, the intermediate frequency signal is converted from optical signal to electric signal which is then demodulated to produce a baseband signal. In the optical receiver, the frequency difference must be stabilized to be constant. For this purpose, an automatic frequency control (AFC) circuit is provided therein to stabilize the intermediate frequency.
In a conventional coherent light communication system, it is necessary to pull an intermediate frequency in a frequency range of the AFC by sweeping a frequency of a local oscillation light beforehand because a frequency of a local oscillation light source is fluctuated much wider than the intermediate frequency, so that the AFC is performed properly.
In the conventional coherent light communication system, however, an intermediate frequency is not automatically pulled in a frequency range of the AFC, but a manual adjustment has been performed. Although only one apparatus in which an intermediate frequency is automatically pulled in a frequency range of the AFC is described in a report entitled "Frequency Acquisition and Tracking for Optical Heterodyne Communication Systems" in "Journal of Lightwave Technology, Vol. LT-5, No. 4, April 1987," the construction thereof is very complicated.
In the conventional coherent light communication system, further, a level of the intermediate frequency signal is fluctuated to result in a deterioration of a receiving property because polarizations of the local oscillation light and the signal light tend to be inconsistent with each other. In order to avoid the inconsistence of the polarizations, a polarization control device must be provided therein so that a level of the intermediate frequency signal is maximized by controlling polarizations of the local oscillation light and the signal light separately from the control of an intermediate frequency as described in "New Polarisation-State Control Device : Rotatable Fiber Cranks" reported in Electronics Letters, Sept. 26, 1985, Vol. 21, No. 20, pp. 895-896 by T. Okoshi.
However, there are further disadvantages that a high frequency stabilization is not obtained because the level fluctuation of an intermediate frequency signal is mixed with a control signal by which a frequency of the local oscillation light is stabilized, and that a system scale becomes large because two systems for stabilizing a frequency of the local oscillation light and for controlling polarizations of the local light oscillation light and the signal light are necessary to be provided therein.